Align GenericWhitespace ws.illegalFollow message with Checkstyle default

[thswlsqls]

The GenericWhitespace module in src/checkstyle/checkstyle-configuration.xml overrides four
messages as "GenericWhitespace " + Checkstyle's default text. Three match the default;
ws.illegalFollow reads "should followed by whitespace" — ungrammatical, and not what the check
reports. It was written that way in #6149, where Checkstyle 9.2.1 already defaulted to
''{0}'' is followed by an illegal character. This aligns the override with that default,
restoring the pattern the other three follow.

Testing done

No automated test: a message-string override has no test surface. Verified
ws.illegalFollow=''{0}'' is followed by an illegal character. in
checks/whitespace/messages.properties of checkstyle 13.8.0, the version pinned in pom.xml.
mvn checkstyle:check -pl cli with the change: BUILD SUCCESS, 0 violations — the ruleset still
parses and flags the same code.

Screenshots (UI changes only)

Before

After

Proposed changelog entries

• N/A

Proposed changelog category

/label skip-changelog

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
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• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
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[MarkEWaite]

Confirmed that the new message matches the checkstyle source code.

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process.

/label ready-for-merge